Adventages of using leaf spring over coil spring

The main difference is the stability we get from leaf springs.  Leaf springs have two distant points of contact which makes them much more stable in heavy trucks.  Where you need more responsiveness than stability, coil springs are the better option. 

Note that McPherson struts in most light cars cut the manufacturing cost by 1/3 over conventional coil and joint steering.  But there is no way to make this technology tough or resilient enough to be practical in an 18-wheeler or of any use in railroad cars.

In short, coils are cheaper and ride better, leaf springs are stronger, more stable, and last 4.75 times longer -- important where safety is the primary factor.